1. Field of the Invention
This invention relates to a melting furnace of a rigid structure.
2. Prior Art
In the construction of a conventional melting furnace having a body made of bricks joined together, it is important that when metal such as copper is melted in the furnace, the molten metal is positively prevented from leaking through the joints between the bricks. For this reason, it is a common practice in the art to provide a melting furnace of a rigid structure. The melting furnace of this rigid structure comprises a shell of iron mounted around a peripheral wall of the furnace body made of bricks joined together. With this rigid construction, the iron shell serves to restrain a thermal expansion of the brick wall during an operation of the furnace to thereby exert predetermined compressive forces on the bricks forming the peripheral wall of the furnace body, so that the joints between the bricks are prevented from breaking or spliting, thereby ensuring that the molten metal in the furnace will not leak through the joints. For the purpose of maintenance of the furnace of the rigid structure, it is often necessary to allow the hot furnace to cool so as to determine whether the furnace needs repair. When the furnace of the rigid structure is subjected to a cycle of heating and cooling repeatedly, the amount of thermal expansion of the bricks forming the furnace body is gradually increased so that the compressive forces exerted by the iron shell on the bricks finally become excessive. This may cause the bricks to fracture under pressure. More specifically, when the furnace of the rigid structure is subjected to one cycle of heating and cooling under a predetermined load, the bricks become smaller due to creep shrinkage after they are allowed to cool to room temperatures. Although the bricks are thermally expanded during the heating of the furnace, they are subjected to such creep shrinkage because they are restrained by the iron shell. As shown in FIG. 1, this creep shrinkage is the total of an amount l.sub.1 of creep shrinkage occuring during a period when the temperature of the bricks rises from a creep-starting temperature T.sub.cr to the maximum heating temperature T.sub.e and an amount l.sub.2 of creep shrinkage occuring during a period when the bricks are maintained at the maximum heating temperature. Thus, the brick becomes smaller by the combined amounts l.sub.1 and l.sub.2 of creep shrinkage. However, in the case where this cycle of heating and cooling is repeated with the bricks under the predetermined pressure in contact with the molten metal such as copper, the molten metal penetrates into the bricks while the bricks are maintained at elevated temperatures. Therefore, as shown in FIG. 2, the amount L' of shrinkage of each cooled brick is smaller than the amount L of its creep shrinkage, and the amount of subtraction of the amount L' from the amount L is present as the residual expansion amount, as indicated by curve 1 in FIG. 2. In addition, when the furnace is heated again as indicated by curve 2 in FIG. 2, the impregnated metal in the brick is also thermally expanded so that a thermal expansion coefficient of the brick is increased. Due to the increase of this thermal expansion coefficient and the residual expansion amount, the maximum amount .lambda. of expansion of the brick immediately before the occurrence of the creep shrinkage in the brick is caused to increase substantially each time the cycle of heating and cooling of the furnace is repeated, as indicated by curves 1 to 3 in FIG. 2. Therefore, even if the compressive forces exerted by the iron shell on the bricks after the furnace of the rigid structure reaches the maximum heating temperature are less than the fracture strength of the bricks due to the occurrence of the creep shrinkage in the bricks, the bricks are subjected to excessive compressive forces greater than the fracture strength during a period when the heating temperature of the furnace rises to its maximum heating temperature. This will result in the fracture of the bricks under pressure, and the melting furnace of the rigid structure will have a shortened service life. In addition, the fracture strength of the bricks is lowered gradually since the bricks are deteriorated with the lapse of time. Due to the lowering of the fracture strength of the bricks and the excessive compressive forces, the bricks forming the furnace body are liable to fracture under pressure, and the service life of the melting furnace of the rigid structure is further shortened.